1. Field of the Invention
This invention generally relates to a cleaning method, and particularly to a method of cleaning optical members such as a lens and a mirror used in an exposing apparatus for manufacturing a device such as a single crystal substrate for a semiconductor wafer or a glass substrate for a liquid crystal display. However, the use of the cleaning method of the present invention is not restricted to the exposing apparatus, but the cleaning method of the present invention can be widely applied to photolithography, projection inspection and optical apparatuses such as a motion picture projector and other projectors.
2. Related Background Art
When manufacturing a minute semiconductor element such as a semiconductor memory or a logic circuit by the use of photolithography technique, use has heretofore been made of a reduction projection exposing apparatus for projecting a circuit pattern depicted on a reticle (or a mask) onto a wafer or the like by a projection optical system to thereby transfer the circuit pattern.
The minimum dimension (resolution) which can be transferred by the reduction projection exposing apparatus is proportional to the wavelength of light used for exposure, and is inversely proportional to the numerical aperture (NA) of the projection optical system. Accordingly, the shorter the wavelength is made, the better the resolution. Therefore, with the requirement for the minuteness of semiconductor elements in recent years, the shorter wavelength of exposure light has been advanced, and an exposure light source has shifted from a super-high pressure mercury lamp (i-line (a wavelength of about 365 nm)) to a KrF excimer laser (a wavelength of about 248 nm) and an ArF excimer laser (a wavelength of about 193 nm), and further, it has been advanced to put an F2 laser (a wavelength of about 157 nm) into practical use.
Light of a wavelength of about 200 nm or shorter becomes very great in light absorption by substances (hereinafter referred to as the “light absorbing substances”) such as oxygen molecules, water molecules and carbon dioxide molecules contained in an optical path atmosphere. Particularly, the ArF laser, the F2 laser, etc. are remarkable in the light absorption by the light absorbing substances, and the transmittance thereof is generally represented by the following mathematical expression 1.Transmittance=exp−(absorption coefficient)×density [ppm]×distance [cm] (atmospheric pressure/atm)  [Mathematical Expression 1]absorption coefficient oxygen: 190 water: 62 carbon dioxide: 9
Therefore, in an exposing apparatus using vacuum ultraviolet light, in order to prevent a reduction in an exposure amount to an object to be processed, an optical member disposed in an optical path is contained in a space purged (or made vacuum) by an inert gas. In an exposing apparatus using an F2 laser as a light source, in order to enable sufficient transmittance to be obtained, it becomes necessary to make the density of the light absorbing substances in an optical path equal to or less than about 1 ppm, and the purge time required for this is shown by the following mathematical expression 2, and a long time and a great deal of inert gas are required to make the light absorbing substances have desired density. So, in order to curtail the purge time and the inert gas required, it has been proposed to divide the space to be purged into isolated chambers, and effect the purge in each isolated chamber.Purge time=purge volume [1]/flow rate of inert gas [1/min]×360 min. to 480 min. or longer  [Mathematical Expression 2]
Also, n an exposing apparatus using a KrF excimer laser, photon energy becomes great and causes impurities and oxygen in an optical path to photochemically react to each other. If a product formed by the photochemical reaction adheres to an optical member, it clouds the optical element, thus resulting not only a further reduction in the exposure amount, but also the deterioration of imaging performance. The impurities include, for example, a halide such as a plasticizer contained in an electric wire coating substance in the optical path, and organic substances or the like contained in working oil, an adhesive agent, etc.
So, in the exposing apparatus, use is made of a non-metal member small in degassing property or a metal member of small surface roughness from which the working oil has been completely removed by ultrasonic cleaning or the like. However, it is not easy to completely remove the impurities, and since clouding occurs to the optical member even during the assembly of the apparatus or during exposure, the cleaning of the optical member after assembly is very important.
The use of plasma, a neutral detergent and organic and inorganic solvents has already been proposed as the cleaning method for the optical member, and in recent years, attention has been paid to light cleaning using ozone for higher cleaning ability. The light cleaning applies ultraviolet light to an optical member to thereby decompose a substance adhering to the optical member. The ultraviolet light produces active oxygen in oxygen gas and activates the substance adhering to the surface of the optical member. Ozone is produced from the active oxygen and oxygen molecules, and when the ozone receives ultraviolet light, they change into active oxygen in its excited state, and decompose and volatilize the substance adhering to the optical member. It is known that the light cleaning becomes low in effectiveness when oxygen molecules are absent.
That is, in the case of vacuum ultraviolet light, to effect exposure with a sufficient exposure amount, the elimination of oxygen molecules which are a light absorbing substance is necessary. On the other hand, in case of light cleaning, there occurs the contradiction that oxygen molecules are necessary. Particularly, in an exposing apparatus using an ArF laser or an F2 laser as a light source, when oxygen or ozone for light cleaning is supplied into an optical path, the laser beam is absorbed by such oxygen or ozone, and this may lead to a case where the laser beam does not reach an optical member at a succeeding stage and therefore light cleaning is not effected. Also, in order to eliminate the oxygen or ozone supplied during the light cleaning, it is necessary to effect the purge of inert gas, and this requires very much time, thus resulting in a remarkable reduction in throughput.